typing: add a couple of assertions to lrucachedict to help pytype Fixes the following warnings: line 1488, in pop: No attribute 'markempty' on None [attribute-error] In Optional[Union[Any, _lrucachenode, nothing]] line 1578, in popoldest: No attribute 'markempty' on None [attribute-error] In Optional[Union[Any, _lrucachenode, nothing]] Differential Revision: https://phab.mercurial-scm.org/D7674